Many optical filters for use in various applications have problems caused by reflection of the filters themselves. For example, in some cases, an optical filter for use in an imaging optical system involves a phenomenon that part of light transmitted through the filter is reflected by another member and re-enters the optical filter from a light emitting surface of the optical filter. In such a case, if the optical filter has a reflectance in a wavelength region of the incident light, the light reflected by another member and re-entered is reflected again by the optical filter. As a result, a problem caused by the light reflected by the optical filter occurs. Therefore, further enhancement of the anti-reflection function of the optical filter has been strongly desired.
A light-absorbing-type optical filter including an absorbent structure, whose surface reflectance is made close to zero as far as possible, can also provide desired transmission characteristics by adjusting light-absorbing characteristics.
In general, as such an optical filter of the type having absorption in a desired wavelength region, an absorption-type ND (Neutral Density) filter for use in a light diaphragm device is widely known.
The following methods have been known for reflection reduction. Patent Literature 1 proposes a method of suppressing the reflectance of any wavelength region, for example, by laminating several types of thin films each having the different refractive indexes and being made of different materials such as SiO2, MgF2, Nb2O5, TiO2, Ta2O5, and ZrO2 to form a multi-layer film-type anti-reflection film. In addition, Patent Literature 2 discloses an ND filter using a fine periodic structure as an anti-reflection structure. Further, Patent Literature 3 proposes a method of improving transmission flatness as an example to obtain the desired spectral reflectance characteristics in a light-absorbing film.